1. Field of the Invention
The present invention relates to pumps, and more particularly to a gimballed mechanical face seal for use in a pump.
2. Description of Related Art
Many appliances used today such as dishwashers, laundry machines, etc., use water pumps that have an impeller that is rotationally driven by a pump motor shaft through rotational locks, such locks can be corresponding flats in the impeller and on the shaft of the motor. However, many of these impeller shaft combinations are free to move axially along the shaft due to radial clearances between the impeller and the shaft. Many of the pumps found in current day appliances use an impeller with a cup shaped recess that fits around but not against the bottom of the end of the motor shaft. This allows substantial clearance between the bottom of the cup shaped recess and the end of the motor shaft and accommodates significant end play of the motor shaft and also accommodates axial tolerances in mounting the pump housing to the motor. The axial positioning of the impeller and the mechanical face seals found within these pumps is independent of the length of the motor shaft and is usually accomplished by spring force of the mechanical face seal holding the impeller against a thrust bearing. The problem with such pumps is that the positive hydraulic pressures developed within the pump work upon a hydraulic area within and effect the hydraulic diameter of the mechanical face seal which results in forces moving the impeller and compressing the seal axially further onto the pump motor shaft.
In prior art appliance pumps movement of the impeller and compression of the seal is prevented by using a spring force in the mechanical face seal which is sufficient to offset the resultant force developed under peak pump hydraulic pressures. The practical pumps of today require a spring force that is two to three times higher than that required for efficient sealing by the mechanical face seal. However, having such high spring forces creates higher than necessary seal wear and frictional heat because most of these pumps operate at considerably less than peak pressure 95% of their operating cycles. However, on the other hand if a high spring load is not provided this will allow the impeller to slide on the shaft of the pump motor compressing the mechanical face seal until the seal is deflected to a solid position. The solid position makes any further seal deflection virtually impossible. With the seal in its solid position it would be unable to accommodate normal misalignments between the pump and motor or misalignments or non-parallel conditions within the pump components without causing non-symmetrical loading and wear of seal components whose flatness is critical in maintaining the sealing function.
Therefore, there is a need in the art for a pump that will allow the use of seal spring loads only sufficient to assure efficient sealing and to allow the impeller to move axially a finite distance.